U.S. patent number 5,535,811 [Application Number 07/007,883] was granted by the patent office on 1996-07-16 for ceramic shell compositions for casting of reactive metals.
This patent grant is currently assigned to Remet Corporation. Invention is credited to Roy C. Feagin.
United States Patent |
5,535,811 |
Feagin |
July 16, 1996 |
Ceramic shell compositions for casting of reactive metals
Abstract
Mold coatings that are relatively unreactive with titanium and
titanium alloys during casting are prepared from yttria slurries,
which may contain other refractory materials, an acid and an inert
organic solvent.
Inventors: |
Feagin; Roy C. (Boca Raton,
FL) |
Assignee: |
Remet Corporation (Chadwicks,
NY)
|
Family
ID: |
21728618 |
Appl.
No.: |
07/007,883 |
Filed: |
January 28, 1987 |
Current U.S.
Class: |
164/139;
106/38.22; 164/519 |
Current CPC
Class: |
B22C
1/00 (20130101); B22C 1/165 (20130101); B22C
3/00 (20130101); C04B 12/00 (20130101); C04B
28/00 (20130101); C04B 35/505 (20130101); C04B
35/584 (20130101); C04B 35/62222 (20130101); C04B
35/6303 (20130101); C04B 35/632 (20130101); C04B
35/66 (20130101); C04B 28/00 (20130101); C04B
14/30 (20130101); C04B 14/30 (20130101); C04B
22/082 (20130101); C04B 28/00 (20130101); C04B
14/30 (20130101); C04B 14/30 (20130101); C04B
24/04 (20130101); C04B 2111/00482 (20130101); C04B
2111/00879 (20130101); C04B 2111/00939 (20130101); C04B
2235/3224 (20130101); C04B 2235/3244 (20130101); C04B
2235/668 (20130101); C04B 2103/0093 (20130101); C04B
2103/0093 (20130101) |
Current International
Class: |
B22C
3/00 (20060101); B22C 1/00 (20060101); B22C
1/16 (20060101); C04B 35/50 (20060101); C04B
35/505 (20060101); C04B 35/66 (20060101); C04B
35/622 (20060101); C04B 12/00 (20060101); C04B
28/00 (20060101); B91D 003/00 (); C04B
035/66 () |
Field of
Search: |
;106/38.22
;164/519,139 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nelson; Peter A.
Attorney, Agent or Firm: Beveridge, DeGrandi, Weilacher
& Young
Claims
I claim:
1. A refractory composition comprising a slurry of yttria, an acid
and an inert organic solvent.
2. A coating composition comprising finely divided yttrium oxide
refractory, an acid, an inert organic solvent and an additional
refractory selected from the group consisting of yttrium oxide,
zirconium oxide, fused yttrium oxide, fused zirconium oxide, fused
stabilized zirconium oxide, a fused homogeneous mixture of yttrium
oxide and zirconium oxide.
3. The composition as set forth in claim 2, wherein the composition
contains a minor amount of water.
4. The composition as set forth in claim 1, further comprising a
sufficient amount of an additional refractory.
5. The composition as set forth in claim 4, wherein the additional
refractory is selected from the group consisting of monoclinic
zirconium oxide, yttrium oxide, cubic zirconium oxide, fused
yttrium oxide, fused zirconium oxide, fused stabilized zirconium
oxide.
6. The composition as set forth in claim 5 wherein fused stabilized
zirconium oxide is present having as the stabilizing agent a member
selected from the group calcium oxide, magnesium oxide, ytrrium
oxide, lanthanum oxide, dysprosium oxide, and oxides of other rare
earth element from the Periodic Table with an atomic number of 57
to 71, blends of zirconium oxide and yttrium oxide or with other
constituents in this group, and fused blends of zirconium and/or
yttrium oxide with other oxides of a rare earth element from the
Periodic Table with an atomic number of 57 to 71.
7. The composition as set forth in claim 2, wherein the acid is an
organic acid or an inorganic acid, that is capable of reacting with
the yttria in the slurry to form a salt that will hydrate in the
presence of moisture.
8. The composition as set forth in claim 2, wherein the solvent is
an alkanol or a ketone.
Description
INTRODUCTION AND BACKGROUND
The present invention relates to ceramic shell molds for the
casting of reactive metals, such as titanium, and process of making
molds. In more detail, the invention pertains to a slurry
containing yttria refractory used as casting compositions to form
shell molds, and processes for preparing same. More particularly,
the invention pertains to a slurry comprising yttria refractory, an
acid, and an organic solvent, for making ceramic shell molds for
casting of reactive metals such as titanium and titanium alloys.
The compositions of the present invention are characterized by
having a desirable shelf life and improved stability. Molds
produced in accordance with the present invention are particularly
useful because they enable the casting of reactive metals with
minimized or essentially no alpha case.
In a further aspect, the present invention relates to ceramic and
foundry cores and the yttria slurries used in making them.
Much effort has been devoted for the past 25 years to providing
compositions and methods for casting reactive metals, particularly
titanium and its alloys, into ceramic molds. This development and
the interest in providing this capability was stimulated by
activity in the nuclear and aircraft industries where it was
necessary to search for high strength and lightweight metals.
Titanium, because of its high strength to weight ratio is sought
after for use in the aircraft industry.
The melting point of titanium metal is almost 3100.degree. F. and,
in the molten condition, reacts with most refractories. Earlier
attempts to cast titanium into ordinary foundry molds were
unsuccessful due to the undesirable chemical reactions between the
hot metal and the surfaces with which it came into contact. For
example, reduction of the silica present in such foundry molds
produced heavy reaction zones on the casting surface. This reaction
layer is known in the industry as "alpha case" and the problems
associated therewith have been described in detail in the
literature.
Machined and formed graphite molds have been used commercially to
make titanium castings. Such molds can be made in such a way as to
minimize the alpha case layer. The U.S. Bureau of Mines has
supported research for almost 25 years on the casting of refractory
metals. Thus, there have been continuing efforts to search for new
materials and methods to reduce or eliminate alpha case.
The use of graphite in investment molds has been described in the
art in such patents as U.S. Pat. Nos. 3,241,200; 3,243,733;
3,256,574; 3,266,106; 3,296,666 and 3,321,005 all to Lirones. Other
prior art which show a carbonaceous mold surface utilizing graphite
powders and finely divided inorganic powders called "stuccos" are
Operhall, U.S. Pat. No. 3,257,692; Zusman et al., U.S. Pat. No.
3,485,288 and Morozov et al., U.S. Pat. No. 3,389,743. These
documents describe various ways of obtaining a carbonaceous mold
surface by incorporating graphite powders and stuccos, various
organic and inorganic binder systems such as colloidal silica,
colloidal graphite, synthetic resin which are intended to reduce to
carbon during burnout, and carbon coated refractory mold surfaces.
These systems were observed to have the disadvantage of the
necessity for eliminating oxygen during burnout, a limitation on
the mold temperature and a titanium carbon reaction zone formed on
the casting surface.
Further developments including variations in foundry molds are
shown in Turner et al., U.S. Pat. No. 3,802,902 which uses sodium
silicate bonded graphite and/or olivine which was then coated with
a relatively non-reactive coating such as alumina. However, this
system still did not produce a casting surface free of
contamination.
Schneider, U.S. Pat. No. 3,815,658 shows molds which are less
reactive to steels and steel alloys containing high chromium,
titanium and aluminum contents in which a mangnesium
oxide-forsterite composition is used as the mold surface.
A number of attempts have been made in the past to coat the
graphite and the ceramic molds with materials which would not react
with the reactive metals being cast. For example, metallic powders
such as tantalum, molybdenum, columbium, tungsten, and also thorium
oxide had been used as non-reactive mold surfaces with some type of
oxide bond. See Brown, U.S. Pat. Nos. 3,422,880; 3,537,949 and
3,994,346.
Operhall, U.S. Pat. No. 2,806,271 shows coating a pattern material
with a continuous layer of the metal to be cast, backed up with a
high heat conductivity metal layer and investing in mold
material.
Basche, U.S. Pat. No. 4,135,030 shows impregnation of a standard
ceramic shell mold with a tungsten compound and firing in a
reducing atmosphere such as hydrogen to convert the tungsten
compound to metallic tungsten or tungsten oxides. These molds are
said to be less reactive to molten titanium but they still have the
oxide problems associated with them.
Brown, U.S. Pat. No. 4,057,433 discloses the use of fluorides and
oxyfluorides of the metals of Group IIIa and the lanthanide and
actinide series of Group IIIb of the Periodic Chart as constituents
of the mold surface to minimize reaction with molten titanium. This
reference also shows incorporation of metal particles of one or
more refractory metal powders as a heat sink material. However,
even those procedures have resulted in some alpha case
problems.
A development by General Electric has provided barrier layers of
refractory oxide in a silica bonded mold for casting alloys
containing significant amounts of reactive metals; see Gigliotti et
al. U.S. Pat. Nos. 3,955,616; 3,972,367 and 4,031,945.
Huseby, U.S. Pat. No. 4,240,828 shows doping a nickel and cobalt
alloy with a rare earth metal and casting into a ceramic mold.
In the 1960's, developments at Wright Air Development Center led to
the formation of a crucible for melting titanium formed from a
titanium enriched zirconium oxide crucible with less reaction to
molten titanium than pure zirconium oxide.
Richerson, U.S. Pat. No. 4,040,845 shows a ceramic composition for
crucibles and molds containing a major amount of yttrium oxide and
a minor amount of a heavy rare earth mixed oxide. Such methods
including the making of a titanium metal enriched yttrium oxide
were only partially successful because of the elaborate and
expensive technique which required repetitive steps.
Molds for casting molybdenum made from zirconium acetate bonded
calcia stabilized zirconium oxide have been made by the Bureau of
Mines.
Feagin, U.S. Pat. No. 4,415,673 discloses a zirconia binder which
is an aqueous acidic zirconia sol used as a binder for an active
refractory including stabilized zirconia oxide thereby causing
reaction and gelation of the sols. Solid molds were made for
casting depleted uranium. A distinction is made in this patent
between "active" refractories and refractories which are relatively
inert. The compositions of Feagin are intended to contain at least
a portion of active refractories. See also Feagin, U.S. Pat. No.
4,504,591.
Adhesive plasters made of a suspension of oxide powder, such as
yttrium oxide and an acid are shown in Holcombe et al., U.S. Pat.
No. 4,087,573. These compositions are described as being
spontaneously hardening and useful for coating surfaces or for
casting into a shape. Of particular interest is the coating of
graphite crucible used in uranium melting operations.
It is generally recognized in the industry that all commercial
processes have some alpha case on their casting. This may range
from about 0.005 inches to 0.04 inches in thickness depending on
process and casting size. The alpha case must be milled off by
chemical means or other means from the casting before a
satisfactory casting is obtained. The extra cost imposed by the
chemical milling operation is a disadvantage and presents a serious
problem from the standpoint of accuracy of dimensions. Normally,
the tooling must take into consideration the chemical milling which
results in the removal of some of the material in order to produce
a casting that is dimensionally correct. However, since casting
conditions vary, the alpha case will vary along the surface of the
casting. This means that there is a considerable problem with
regard to dimensional variation.
Some refractory compositions have been developed that exhibit
reduced alpha case and can be used successfully to make production
castings by applying the coatings to the wax patterns by special
techniques, such as spraying. However, a difficulty arises in that
certain refractory mixes do not have a long pot life and gel
quickly, even spontaneously with stirring in a few minutes,
depending upon exact composition. See Holcombe et al., U.S. Pat.
No. 4,087,573.
Accordingly, it would be highly advantageous to have a slurry that
is stable at least for several days and preferably for several
weeks in order that patterns of the desired shape may be dipped
into the slurry according to present production practice.
SUMMARY OF THE INVENTION
The present invention pertains to yttria used in making ceramic
shell molds, and in particular to slurry compositions comprising
yttrium oxide refractory, an acid, and an organic inert solvent.
The yttria refractory can be either fused or unfused, and can be
present alone, or along with some other refractory if desired. It
is preferred that the yttria be fused and ground to appropriate
flour size, but unfused yttria or sintered yttria can also be used.
The fused refractory allows for somewhat higher filler loading
which decreases the tendency toward minute cracks on firing. Blends
of yttria, fused or unfused, with other refractories such as
zirconium oxide can be employed in making a mold coating
composition.
The invention further relates to the reactive compositions of the
yttria refractory, optionally with other refractory materials, and
the mold coatings and cast shapes prepared therefrom. The invention
also pertains to the method of making the coating compositions and
methods of making molds. The invention is of particular interest
with respect to the casting of titanium and titanium alloys such as
Ti6A14V.
In a further aspect of the invention, the invention pertains
particularly to coatings and molds, ceramic and foundry cores for
metal casting, and to the use of a slurry of yttria that has good
stability and can contain a variety of other refractories, metal
powders, and fibers for various applications and purposes.
An object of the invention is to provide a stable composition
suitable for making a mold coating which is less reactive with
titanium and titanium alloys during casting.
Another object of the invention is to provide a low reactivity mold
coating which is characterized by good stability for reactive metal
casting.
A further object of the invention is to provide a ceramic mold
having a casting surface having low reactivity with reactive
metals.
A still further object of the present invention is to provide a
process for making an investment casting mold having low reactivity
with reactive metals.
A further object of the invention is to provide a ceramic shell
mold with a relatively reaction-free surface against which metals
may be poured.
Yet another object of the invention is to provide a castable
refractory composition of good stability, using a slurry of yttria,
and the resulting ceramic body.
A still further object of the present invention is to provide a
process for making an investment casting mold having low reactivity
with molten titanium, titanium alloys, zirconium and zirconium
alloys.
A further object of the present invention is to provide a coating
composition suitable for spraying or painting of a foundry mold,
foundry and ceramic cores, melting crucible, ladle, or pouring
basin to make it more resistant to reactive metals.
A still further object of the present invention is to provide
ceramic and foundry cores utilizing a slurry of yttria.
As used herein, the term "reactive metals" means those metals and
alloys which may react with or produce a relatively rough mold
surface when poured into ordinary investment casting molds having
mold surfaces containing one or more of the following refractories:
silica, alumina, aluminosilicates, zirconium silicate (zircon) or
other oxides and mixed oxides normally used in investment casting
molds. Examples of these reactive metals are titanium, titanium
alloys such as Ti6A14V, zirconium, zirconium alloys, high carbon
steels, eutectic alloys (containing appreciable amounts of
tungsten, hafnium, carbon, nickel, cobalt, etc.), aluminum-lithium
alloys, nickel base alloys containing appreciable amounts of
titanium or aluminum or hafnium or tungsten. The reactive metals
are well known in the art and one of the most reactive of all of
these metals is titanium.
In accordance with the present invention, there is provided a
slurry or suspension type composition containing yttria in finely
divided form, either fused or unfused, as the main component. Any
additional compatible finely divided refractory can be blended with
the yttrium oxide. In particular, suitable refractory materials for
this purpose include zirconium oxide, fused zirconium oxide,
monoclinic zirconium oxide, cubic zirconium oxide, fused stabilized
zirconium oxide having as the stabilizing agent a member selected
from the group consisting of calcium oxide, magnesium oxide,
yttrium oxide, lanthanum oxide, dysprosium oxide, and other rare
earth oxides, blends of zirconium and other constituents in this
group, and fused blends of zirconium with other rare earth oxides,
and mixtures of any of the above. Generally, the yttria is present
as the major refractory component when blended with other
refractory materials as mentioned above. The term "rare earth" is
used herein to denote a member of the Periodic Table of Elements
with an atomic number of 57 to 71.
In a further aspect, the invention resides in a process for making
an investment casting mold having low reactivity with reactive
metals comprising providing a stable slurry composition or
suspension of yttria refractory, organic solvent and acid, applying
the slurry to a pattern, exposing the coated pattern to appropriate
conditions of moisture to facilitate the formation of a "green
bond" and thereafter firing to produce a fired bond.
A still further feature of the invention resides in a process for
making a ceramic shell mold comprising coating the outside of a
pattern with the afore-described slurry comprising yttria, acid,
solvent and any of the above-mentioned additional refractory
materials, drying and heating and then firing to form the intended
shell mold. In an alternative procedure, before the coating is dry,
there is applied thereto a finely divided refractory called a
"stucco". The art of stuccoing is well understood. After the stucco
is applied, the coating is allowed to dry, the shell is removed
from the pattern and fired at a sufficiently high temperature to
bond the refractory together.
Also, a further feature of the invention resides in a process for
casting a reactive metal in a mold having low reactivity with
reactive metals comprising providing a stable suspension of yttria
refractory, organic solvent and acid, optionally with other
refractory material and/or some water to form a coating
composition, applying said coating composition to a pattern shaped
in the desired configuration, allowing the coating react with
moisture to gel, heating the resulting coated pattern to a
sufficiently high temperature to fire and thereby fusing said
coating composition into the desired shape, and thereafter casting
said metal into the said desired mold.
A still further feature of the invention resides in a process for
making a core wherein a suitable core mold is coated with a slurry
comprising yttria, acid and organic solvent. After coating and
while the coating is still wet, it can be stuccoed by application
of a suitable refractory such as for example fused yttrium oxide
and thereafter permitted to dry. After drying a castable mix of any
compatible refractory is added to the core mold having the coating
applied thereto and the mix is permitted to gel. After gelation,
the core can be removed from the mold and fired to bond the coating
to the cast back up to thereby form a finished core.
DETAILED DESCRIPTION OF THE INVENTION
The compositions of the present invention can be characterized as
suspensions comprising yttria refractory, an inert organic solvent
and a suitable acid. It has been found that a small amount of water
can be tolerated in these compositions. Typically, the acid used
for purposes of the invention, for example, HCl, HNO.sub.3, etc.,
will contain an appreciable amount of water even though the acids
are used in small amounts in relation to the total amount of
slurry. In carrying out the invention, care should be taken to
control the amount of water as the more water that is present, the
shorter will be the shelf life ("pot" life) of the slurry. Under
ideal conditions, no water should be added to obtain long-time
shelf life of the slurry; i.e. up to several months of stability.
However, the slurries of the invention can tolerate some water
especially if it is desired to use the slurry quickly. Also, such
factors as amount of exposure to the atmosphere, particle size of
the yttrium oxide refractory, amount of acid used, and whether one
wishes a fast setting slurry will influence the water content. If
the slurry is to be used quickly as for spraying or drying, then
more water should be added so that some setting takes place rapidly
during the drying operation and to be less dependent on the
humidity conditions of the atmosphere. These are matters that will
be apparent to persons skilled in the art after a starting of this
specification.
The yttria slurry is prepared by mixing the components thereof in
any convenient manner using conventional equipment. The yttria is
in finely divided form, sometimes called "flour". The term "flour"
is commonly used in the foundry industry to signify the finely
ground refractory materials that are commonly used to prepare
slurry compositions. Particle sizes can vary considerably and still
be suitable for the intended purposes. In general, however, in the
investment casting industry, it means particle sizes below 150
microns can designate sizes down to 1 to 10 microns. A common flour
size used in the industry is a flour containing particles
essentially 75% finer than 325 mesh (44 microns) and usually has a
wide distribution range. The "mesh" sizes refer to U. S. Standard
Screen Series.
Another commonly used size in the industry is generally referred to
as 325 mesh flour and is understood to mean that at least 95% of
the particles pass through a 325 U.S. Standard Screen mesh. In this
instance, 95% of the flour particles are finer than 44 microns.
Commonly used flours are -150 mesh, -200 mesh and -325 and this is
understood to mean that the particles are sufficiently ground so
that at least 96% of the particles pass through the designated
screen. The distribution of particle size ranges is not generally
provided but, can be determined by means known in the art.
Producers of such flours have been known to provide what is called
a "typical" screen analysis of a particular grade with no guarantee
that each lot will be the same or conform to the analysis of the
"typical" batch. Essential for purposes of the invention is that
the flour be of sufficiently fine particle size so that a smooth
surface will be provided on the mold.
During the slurry preparation, the acid present will react with
some of the yttrium oxide to form an yttrium salt of the acid. This
salt then reacts in a manner that is similar to that of calcium
sulfate when exposed to moisture; namely, it forms a hydrate of the
salt which constitutes the "green bond" when the coated pattern is
exposed to a moist atmosphere. This hydrate then dehydrates upon
firing to form yttrium oxide which becomes the fired bond.
For purposes of the present invention, refractory powders or
aggregate suitable for use in addition to the yttrium oxide for the
mold compositions and coatings are those of the following group:
monoclinic zirconium oxide, yttrium oxide, cubic zirconium oxide,
fused yttrium oxide, fused zirconium oxide, fused stabilized
zirconium oxide having as the stabilizing agent a member from the
group of calcium oxide, magnesium oxide, yttrium oxide, scandium
oxide, the oxides of the lanthanides of the Periodic Table; e.g.
lanthanum, cerium, dysprosium, praseudymium, neodymium, samarium,
and other rare earth oxides, blends of zirconium and yttrium oxides
or with other constituents in this group and fused blends of
zirconium and/or yttrium oxide with other rare earth oxides. These
may be used in with the yttria in the slurry, or mixtures of one or
more thereof may be used.
In accordance with the invention, the slurry also contains as an
essential ingredient an acid. The acid used may be any acid that
will react with the yttria under normal room temperature conditions
to form a salt that will, in turn, form a hydrate. The salt
preferably should be totally soluble in the solvent, but partial
solubility is satisfactory. The acid also should be essentially
soluble in the solvent and should be relatively stable under
ambient conditions except for any reaction with the yttria
refractory. Examples of suitable acids are organic mono- and
polycarboxcylic saturated acids such as formic, acetic, propionic,
citric, succinic, oxalic, tricarballyllic, phthalic, maleic and
tartaric. Inorganic acids include sulfuric, nitric, hydrochloric
and sulfamic. Others may also be used. The concentration of the
acid must not be too great so as to appreciably dissolve the
yttrium oxide or other refractory substances; it must also not be
dilute with water as that will contribute too much water to the
system.
The solvent that is present in the slurry in accordance with the
invention should preferably be one that does not have a high vapor
pressure at room temperature which may cause cooling and possible
cracking of the wax pattern upon applying the slurry. It should
also not have an extremely low vapor pressure, taking days to dry
on the wax pattern. The solvent preferably should be water soluble
or partially so to facilitate the hardening of the coating.
Solvents that can be used in accordance with the invention are
generally inert, organic solvents including but not limited to
ketones, lower alkanols and esters such as acetone, methyl isobutyl
ketone, methanol, ethanol, butanol, isobutanol, n-propanol,
isopropanol, hexanol, methyl ethyl ketone, ethyl acetate, methyl
acetate, isopropyl acetate, 1,4-dioxane, ethoxy ethanol
(cellosolve), methoxy ethanol (methyl cellosolve), methoxy
isopropanol, and others. Blends of solvents can also be used.
Higher boiling organic solvents, such as propylene glycol
monomethyl ether, carbitol, may also be used in whole or in part
but drying time of the coating is extended.
Yttrium salts have the characteristic of forming a hydrate or
hydroxysalt in the presence of water. (New Yttria Plasters, C. E.
Holcombe, et al., U.S. Department of Energy, Report Y-2104, January
1978). For example, yttrium oxide, when mixed with a dilute acid,
such as nitric acid, to form a slurry will gel up into a hardened
condition within a short period of time, generally within several
minutes.
Because of the reaction of the yttrium salt with water and
resulting in a short pot life a slurry can be prepared using a
solvent containing no H.sub.2 O or having the H.sub.2 O at a
minimum so that the slurry will be stable for days and weeks. When
water is used as the suspending medium the slurries gel or "set"
very quickly within minutes or a few hours making it unsuitable for
some purposes. The wax patterns may be dipped into the slurry and
exposed to the atmosphere which contains some moisture. The
moisture in the atmosphere will enter into the reaction with the
yttrium salt formed to form a hydrated salt and cause hardening of
the coating on the pattern. It may be preferable to expose the
moist dipped pattern instead to a controlled high humidity
atmosphere and then drying to produce stronger coatings and to have
better production control. This technique enables a stable slurry
to be used for dipping patterns and then to harden the coating on
the pattern after dipping and treatment with stucco.
It may be advantageous to include in the slurry certain other
materials such as plastic latexes, film-formers, soluble plastic
materials, organic or inorganic fibers, other refractory fillers,
etc. The inclusion of certain film formers or plastic materials may
be desirable to minimize or prevent penetration of backup slurry
media through the first coat or to minimize any spalling or
cracking during dewaxing of the finished ceramic shell. Such
additives and adjuvants are well known in the art for this
purpose.
In another embodiment of the invention, it is desirable to include
an yttrium salt in the slurry for greater control of the setting
action of the coating. The yttrium salt should be soluble in the
solvent and can be used in place of the yttrium oxide and acid
combination. Thus, the yttrium salt can replace all, or part of,
the yttrium oxide and acid in the slurry composition. The mixture
of the yttrium salt is set forth above and one that is capable of
forming a hydrate with moisture. Examples of such salts are the
acetate, nitrate, chloride, sulfates and any other salt capable of
forming a hydrate with water.
A number of experimental slurries were made to determine shelf life
of the slurries. These are shown in Tables 1 and 2. These tables
illustrate a number of examples of types of solvents and acids
used. Unfused Y.sub.2 O.sub.3 powder (made by precipitation and
drying) and sintered Y.sub.2 O.sub.3 powders were also used. The
influence of water additions is noted on the shelf life of the
slurry. The presence of water reduces the shelf life and therefore
the amount of water should be controlled as previously
indicated.
With regard to the yttria powder used in the examples, two types of
powder were used. One was obtained from the producer and used as
such. The other is a highly sintered powder adjacent to a fused
yttrium oxide ingot. The sintered powder is densified from that
received from the producer, but is not densified completely to a
fused product. Fused ingots were made which were crushed and ground
to appropriate sizes for later experiments. Any suitable yttria
powder can be used for purposes of the invention such as are
commercially available.
TABLE 1
__________________________________________________________________________
R08402 Formula No. -37 -38 -39 -41 -42 -48 -53 -17
__________________________________________________________________________
Composition: Y.sub.2 O.sub.3 Powd. 25 25 25 25 25 25 25 40
Isopropanol 99.9% 25 25 25 25 25 25 25 25 Citric Mono- 2.5 1.25
15.0 17.5 Hydrate Acid 85% Lactic Acid 2.5 Stability 6 mo. 6 mo. 6
mo. 6 mo. 6 mo. 6 mo. 6 mo. 6 mo. Glac. Acetic 10.0 Conc. Nitric
10.0 5.0
__________________________________________________________________________
TABLE 2
__________________________________________________________________________
Formula No. -75 -76 -77 -78 -81 -82 -60 -61 -88 -85 -11 -89
__________________________________________________________________________
Composition: Y.sub.2 O.sub.3 Powd. 25 25 25 25 40 40 40 40 50
Sintered 3A Ethanol 25 25 25 20 20 Conc. Nitric 5 5 Acid Anhyd.
Citric 5 5 10 Acid Citric 5 11.6 11.6 18.5 11.6 18.5 Monohydrate
Acid Dowanol PM 5 25 25 Dist. Water 1.5 1.5 Stability 3 days 2 wks
2 wks 3 days 10 days 2 wks 2 mo. 2 mo. 2 mo. 10 days 3 wks 2 mo.
Isopropanol 25 25 20 25 20 Glacial Acid 2.5 Acetic Y.sub.2 O.sub.3
Powder 25 25 25 Methanol 7.5 7.5
__________________________________________________________________________
A number of the slurries were deposited on a sheet of wax, the
excess slurry drained off, and exposed to the atmosphere having a
humidity above 50% and in most cases as high as 75-80%. These were
dried overnight and examined for film strength. Slurries 37, 38,
39, 41, 17, gave some films that were abrasion resistant with 17,
25, 39, 41 being quite hard and strong. Films from -60 and -61
required extensive drying times to acquire strength. Film strengths
on 75, 76 and -77 were very strong and abrasion resistant as were
81 and 82. Films from 88 and 89 were very hard and strong after
complete drying.
A number of slurries were used to deposit films on wax and
immediately after deposition the wax sample with film was placed
into a container above water which produced a high humidity
atmosphere. These films were quite hard after drying.
In preparing molds for casting in Ti6A14V alloy a number of pattern
wax bars approximately 1/2" to 5/8" in diameter were cut into
lengths (fingers) about 4"long. The application of the coating was
by dipping the finger into the slurry or spraying the coating on to
the wax finger leaving about 1/2 inch of the finger free. After
dipping and draining the moist coating was stuccoed with a coarse
fused yttrium oxide of approximate particle size of -40+140 mesh.
The coating was then allowed to dry in the atmosphere which was
above 50% relative humidity. After drying the pattern was then
sealed to a central wax sprue along with several other experimental
coated fingers. The uncoated wax sprue was then painted with a
fused pure yttrium oxide slurry in colloidal yttria and stuccoed
with the fused yttria stucco. It was allowed to dry and then dipped
into a backup slurry and stuccoed. The back-up slurry is composed
of 30% aqueous silica sol mixed with Remasil 60, 325 mesh to a
viscosity of 25 seconds #4 Zahn cup. Seven back-up coats were used.
This coating was allowed to harden and was then recoated with the
same slurry, and then stuccoing with Remasil 60 having a 50 mesh
grain size. This was repeated with the 50 mesh grain size stucco
until the total number of coats had been applied to the pattern.
The last coating was not stuccoed. After all coats were applied,
the molds were allowed to dry thoroughly for several days before
dewaxing, although a long dry time is not essential. Remasil 60 is
an alumino-silicate refractory of approximately 60% Al.sub.2
O.sub.3 content supplied by Remet Corporation of Chadwicks, New
York.
After the final coat was applied, usually 6-8 coats, no stucco was
applied to the final coat, the mold was allowed to dry at room
temperature. It was then immersed in hot motor oil to remove the
wax leaving a shell mold. The resulting shell mold was fired to
2500.degree. F. for two hours and cooled to room temperature.
As a variation of each of the facecoats, a small amount of an
acrylic vinyl latex could be added to the slurry to aid in
continuous film formation, better adhesion to the wax bar, and to
prevent possible penetration of the silica binder to the mold
surface during dipping of the backup coats. Several known types of
latex can be used if compatible with the slurry.
The refractories and other materials that can be used in preparing
the slurries used in accordance with the invention are described as
follows:
1. Vinyl-acrylic latex, a commercial product available from several
sources; e.g. Air Products Co., used to provide improved film
forming properties to the slurry and to prevent penetration of
liquids from subsequent coats to the mold surface.
2. A low-foaming wetting agent such as Sterox NJ a product of
Monsanto Chemical Co. can be used.
3. 2-ethyl hexanol, a commercial chemical, can be used as a
defoaming agent.
4. Glacial acetic acid, a standard commercial product.
5. Fused yttrium oxide, made by electrically fusing a 99.9% Y.sub.2
O.sub.3 powder and grinding to 200 mesh powder showing 1.9%+200
mesh.
6. Yttria stucco is the same product as 5. but -40+100 mesh
particle size.
Each mold was then attached to a sprue connector in the casting box
to a centrifugal casting machine. Foundry sand with a minor sodium
silicate bond was rammed around the molds in the box. After drying,
the box was put under vacuum and degassed in a large chamber and
molten commercial titanium alloy 6A14V was poured into the molds
under vacuum.
After casting, the molds were cooled, fingers were cut off,
sectioned, embedded in plastic and metallographically polished,
etched to show up the alpha case, and examined microscopically for
alpha case.
Table 3 shows the slurry formulations used for the mold surfaces
for each pattern cast.
Table 4 shows the average alpha case measurements on each finger
casting. These are very low alpha case values.
TABLE 3
__________________________________________________________________________
SLURRY FORMULATIONS Pattern SA SB SC SD SJ SK SL
__________________________________________________________________________
Slurry No. 422 423 424 425 460 461 462 Composition: Citric Acid
Monohydrate 3.125 5.8 3.125 5.8 5.8 5.8 5.8 99% Isopropanol 12.5
12.5 12.5 12.5 5% Nirez Resin 12.5 10% Nirez Resin 12.5 15% Nirez
Resin 12.5 Unfused Y.sub.2 O.sub.3 Powder 13.4 13.4 Approx. 325
Mesh Fused 75 75 40.2 40.2 70 70 70 Y.sub.2 O.sub.3
__________________________________________________________________________
TABLE 4 ______________________________________ AVERAGE ALPHA CASE
DEPTH MEASUREMENTS Sample Case Depth .times. 0.0001"
______________________________________ SA 9 SB 13 SC 12 SD 21 SJ 12
SK 9 SL 9 ______________________________________
On Table 4, it is noted that samples SA, SK and SL had very low
alpha case. Samples SB, SC and SJ were also good but contained
somewhat higher alpha case. Sample SD is higher and may be due to a
process defect unknown at this time.
Relative to the proportions of refractory in yttria slurry, in the
case of relatively "inactive" refractories such as monoclinic
zirconia, tabular alumina, fused silica and zircon, these can vary
widely depending upon their particle size distributions, the
specific gravity of the refractory, the manner of processing such
as injection molding, casting, pressing or dipping, and the
application of the mix. In general, when slurries are made for
dipping investment casting patterns with fused yttria refractory
flour of about 325 mesh, a ratio of one part yttria flour to one
part vehicle is about the minimum. As much as two parts of
refractory can be used to produce a thick slurry coat. Variations
in these proportions may be made depending upon the particular
results desired.
It is further advantageous to include refractory fibers with the
yttria slurry to produce high temperature refractories. Such fibers
include silicon carbide, silicon nitride, carbon fibers, alumina
fibers and the like.
Many of the compositions produced from the yttria slurry refractory
fiber system may be used for special coatings and for casting
shapes when a gelling agent or active refractory is used with the
yttria slurry.
The separate slurries made with yttria powder, zircon, alumina and
fused silica were deposited on a wax pattern and allowed to dry.
The resulting coating was strong and resisted scrapping with a
knife. Protective coatings may be applied to many types of surfaces
such as ceramics, metals, foundry molds, and for electronic
applications. For example, a slurry of citric acid, isopropanol and
fused yttria can be used to spray paint or coat a refractory
melting crucible to minimize metal crucible reaction. It may also
coat a pouring basin or ladle to minimize reaction. In particular,
a thin surface layer may be sprayed on to a cope and drag foundry
mold and dried to form a strong non-reactive coating for protection
when reactive metals, such as titanium are poured into the mold. A
single layer is usually sufficient to give good protection.
A preformed ceramic casting core suitable for casting molds for
titanium casting was made by coating the inside of the core mold
with, for example, a slurry composed of fused yttrium oxide, citric
acid and isopropanol to a viscosity of about 20 seconds #4 Zahn
cup. A few drops of a non-ionic wetting agent, Sterox N.J., can be
added to facilitate wetting of the mold surface. After coating and
while the coating is wet, it can be stuccoed with a -40+100 mesh
fused yttrium oxide grain. The coating can then be allowed to dry
at room temperature. After drying a heavy castable mix of a fused
silica refractory of varying particle size distribution from 20
mesh and down and a prehydrolyzed ethyl silicate binder containing
20% SiO.sub.2 and some ammonium carbonate gelling agent can be
added to the core mold with the above coating and the mix allowed
to gel. After gelation, the core can be removed from the mold. It
can then be fired sufficiently high to bond both the coating and
the cast backup to form a finished core ready for casting.
The refractory mix suitable for the bulk of the core may be any mix
that will be compatible with the coating and is primarily used to
provide a backing or support for the coating or core surface. If
titanium or other reactive metal is poured against the core, the
refractory might be yttria. If the metal is less reactive than
titanium, it may be alumina, zirconia or some other refractory.
A ceramic core suitable for reactive metal casting and particularly
titanium can be made by making a yttria type refractory or other
and casting or injection molding or pressing into a suitable mold
and allowing the slurry to gel. The gelled body can then be dried
and fired and used as a casting core.
Further variations and modifications will be apparent to those
skilled in the art from a reading of the foregoing and are intended
to be encompassed by the claims appended hereto.
* * * * *